A new and improved audio pop fix from KB1GMX

Allison KB1GMX has come up with an improved pop fix based on the one in the v4 board design, originally submitted to the BITX20 list by Joe VE1BWV.

Allison finally got annoyed enough by the pop to fix it.

Parts count 5:

  • 2x 1n4148/914 diodes
  • 10K resistor (any value from 10 to 100K really)
  • 2n7000 MOSFET
  • 0.1 µF capacitor

Allison has added a second diode.   Why? The TX line is relay switched  and relays take milliseconds to physically move contacts. So the second diode to the T/R line from the Raduino is the fast acting “audio kill”. The second diode and parallel resistor is the hold until the relay returns to RX position.

Allison assures us that this fully mutes the rig with no pop, no thump in either transition (from RX to TX or TX to RX).

Michael VE3WMB points out that connecting to VOL-H will kill the sidetone output.   He notes that Ashhar Farhan VU2ESE has his V4 pop circuit connected at M2(R70) with the value of R70 increased to 1K ohms in order to hear the CW sidetone.


Research into the audio pop problem

John KK5VH has been working on understanding the audio pop problem for a while. He doesn’t have a fix yet, but he has identified that:

  1. The relays have a max actuation time of 7ms.
  2. K1 drives K3 via the TX voltage –  that means you have a 7ms (max) time from when the TX turns on and K3 disconnects the audio.
  3. Changing the power (turning RX/TX  on and off) on these one transistor amplifiers will cause a large spike to be coupled via C63 and C51 to the audio IC before K3 can cut them off.
  4. Both the amplifiers draw tiny amounts of current, Q6 about 1.3ma and Q70 about 2.2ma.  They do not have enough current draw to bring their respective power sources down quickly. Secondly, they have 47µF capacitors that hold the power up within that circuit.

John has been simulating the circuit via LTspice with some results.

He increased C64 to 517µF by paralleling a 470 µF cap across it and changed C52 from 47µF to 0.1 µF.  That made a timing difference that cured the turn on pop but left a gigantic pop on turning off the rig or moving from TX back to RX.

After all of this playing around he still don’t have a good hardware answer to the problem!

However, John suggests that if the Arduino Nano controlled K3, this could solve the problem using timing delays. A simple sequence would in moving from RX to TX, first turn on K3 (disconnecting the audio chain) then switch on K1 into TX mode.  When finished with TX mode, hold K3 on for a number of milliseconds to all the RX circuits to stabilize before switching back to RX on K1. Hopefully, this would solve the problem.  John welcomes comments!